In the synthesis of polyalkenes such as polyethylene and polypropylene, solid particles of polymer product are withdrawn from the reactor with interparticle gaseous components present in the reactor. The solid polymer product is purged with an inert gas such as nitrogen before the product is pelletized, yielding a nitrogen-rich purge gas containing a number of hydrocarbon compounds including unreacted ethylene or propylene, some alkanes such as ethane or propane, possible co-monomers such as butene-1 and hexene-1, additives such as isobutane, isopentane, or isohexane which may be added to the reactor feed as diluents or moderators, and possibly small amounts of oligomers such as octene, decene, or heavier olefins. The hydrocarbon content of this product purge gas can range up to 50 mol % or higher, most of which consists of the valuable feed components ethylene or propylene. Recovery of these feed components and additional hydrocarbons utilized in the polymerization process is economically desirable. Heavier components such as oligomers, if present in the purge gas, may be detrimental to adsorption or low temperature condensation methods for recovering the valuable lighter components.
Improved methods for the recovery of ethylene or propylene from nitrogen-rich purge gas in the production of polyethylene or polypropylene are economically desirable. The presence of heavier hydrocarbons in the purge gas may adversely affect adsorption processes used for light hydrocarbon recovery, and methods for removing such heavy hydrocarbons can be beneficial for improved recovery of valuable ethylene or propylene monomers. The integrated process disclosed below and defined in the claims which follow addresses these problems and offers improved purge gas treatment for the recovery of alkene reactants for recycle to the polymerization reactor system and nitrogen for reuse as purge gas.
The invention relates to a method for the separation of a gas mixture which comprises:
(a) obtaining a feed gas mixture comprising nitrogen and at least one hydrocarbon having two to six carbon atoms;
(b) introducing the feed gas mixture at a temperature of about 60xc2x0 F. to about 105xc2x0 F. into an adsorbent bed containing adsorbent material which selectively adsorbs the hydrocarbon, and withdrawing from the adsorbent bed an effluent gas enriched in nitrogen;
(c) discontinuing the flow of the feed gas mixture into the adsorbent bed and depressurizing the adsorbent bed by withdrawing depressurization gas therefrom;
(d) purging the adsorbent bed by introducing a purge gas into the bed and withdrawing therefrom an effluent gas comprising the hydrocarbon, wherein the purge gas contains nitrogen at a concentration higher than that of the nitrogen in the feed gas mixture;
(e) pressurizing the adsorbent bed by introducing pressurization gas into the bed; and
(f) repeating (b) through (e) in a cyclic manner.
The adsorbent bed may be one of a plurality of parallel adsorbent beds wherein each bed of the plurality of beds may be subjected in turn to (b) through (f). The purge gas of (d) may be provided by a portion of the depressurization gas from another adsorbent bed. A portion of the pressurization gas of (e) may be provided by a portion of the depressurization gas from another adsorbent bed. A portion of the pressurization gas of (e) may be provided by a portion of the effluent gas from another adsorbent bed. The plurality of parallel adsorbent beds may consist of four adsorbent beds.
The at least one hydrocarbon having two to six carbon atoms in the feed gas mixture may comprise at least one saturated hydrocarbon containing from two to four carbon atoms. The at least one hydrocarbon having two to six carbon atoms in the feed gas mixture may comprise at least one unsaturated hydrocarbon selected from ethylene and propylene. The adsorbent material may comprise silica gel, activated alumina, or silica gel and activated alumina. The temperature of the feed gas mixture may be maintained in a range of about 70xc2x0 F. to about 95xc2x0 F.
The feed gas mixture may be obtained from a polyalkene product purge gas which is partially condensed and separated into a product purge vapor and at least one product purge liquid, wherein the product purge vapor provides the feed gas mixture into the adsorbent bed.
In another embodiment, the invention relates to a method for the separation of a polyalkene product purge gas containing nitrogen and at least one unsaturated hydrocarbon selected from ethylene and propylene, which method comprises:
(a) compressing the polyalkene product purge gas to yield a compressed product purge gas;
(b) cooling the compressed product purge gas to yield a partially condensed product purge stream;
(c) separating the condensed product purge stream into a product purge vapor and a product purge liquid;
(d) warming the product purge vapor stream to yield a warmed product purge vapor stream; and
(e) separating the warmed product purge vapor stream in a pressure swing adsorption process to yield a purified nitrogen product gas and a purified hydrocarbon product gas containing at least one unsaturated hydrocarbon selected from ethylene and propylene.
The method may further comprise recycling the purified hydrocarbon product gas by combining it with the polyalkene product purge gas prior to compression in (a). The cooling of the compressed product purge gas to yield a partially condensed product purge stream may be effected by
(1) dividing the compressed product purge gas into a first product purge gas stream and a second product purge gas stream;
(2) cooling the first product purge gas stream by indirect heat exchange with the product purge vapor stream to yield a first cooled and partially condensed purge stream;
(3) cooling the second product purge gas stream by indirect heat exchange with a vaporizing process stream to yield a second cooled and partially condensed purge stream; and
(4) combining the first and second cooled and partially condensed purge streams to yield the partially condensed product purge stream of (b).
The product purge liquid of (c) may be reduced in pressure to provide the vaporizing process stream of (3). The vaporizing process stream may be completely vaporized to yield a recovered product gas stream containing at least one unsaturated hydrocarbon selected from ethylene and propylene.
The compressed product purge gas may be cooled by indirect heat exchange with cooling water prior to dividing the compressed product purge gas. The second product purge gas stream may be cooled by indirect heat exchange with cooling water after dividing the compressed product purge gas. The temperature of the warmed product purge vapor stream may be maintained in a range of about 60xc2x0 F. to about 105xc2x0 F. If desired, the temperature of the warmed product purge vapor stream may be maintained in a range of about 60xc2x0 F. to about 105xc2x0 F. by controlling the flow rate of the first product purge gas stream. More specifically, the temperature of the warmed product purge vapor stream may be maintained in a range of about 70xc2x0 F. to about 95xc2x0 F.
The invention also includes a system for the separation of a gas mixture containing nitrogen and at least one hydrocarbon having two to six carbon atoms, which system comprises:
(a) compression means for compressing the gas mixture to yield a compressed gas mixture;
(b) cooling means for cooling the compressed gas mixture to yield a partially condensed stream;
(c) separating means for separating the partially condensed stream into a vapor stream and a liquid stream;
(d) heat exchange means to warm the vapor stream to yield a warmed vapor stream; and
(e) a pressure swing adsorption system for separating the warmed vapor stream to yield a purified nitrogen product gas and a purified hydrocarbon product gas comprising at least one hydrocarbon which contains two to six carbon atoms.
The cooling means for cooling of the compressed gas mixture to yield a partially condensed stream may comprise
(1) piping means to divide the compressed gas mixture into a first gas stream and a second gas stream;
(2) indirect heat exchange means for cooling the first gas stream by indirect heat exchange with the vapor stream of (c) to yield a first cooled and partially condensed stream;
(3) indirect heat exchange means for cooling the second gas stream by indirect heat exchange with a vaporizing process stream to yield a second cooled and partially condensed stream; and
(4) piping means for combining the first and second cooled and partially condensed streams to yield the partially condensed stream of (b).
The system may further comprise piping means for recycling the purified hydrocarbon product gas and combining it with the gas mixture prior to the compression means of (a). In addition, the system may further comprise flow control means to control the flow rate of the gas stream of (1).